Imagine a solar panel. It doesn't just sit there — it has to absorb photons and convert that energy into something useful. In photosynthesis, the "solar panels" are photosystems. But there's a catch: a single chlorophyll molecule can only absorb a photon and get excited, but it can't do much with that energy on its own. It needs help.
So nature builds a complex — a cluster of hundreds of pigment molecules (chlorophylls and carotenoids) surrounding a special pair of chlorophyll molecules at the reaction centre. The surrounding pigments act like an antenna: they absorb light from different wavelengths and funnel the energy inward, like a crowd passing a message to the stage. This is called resonance energy transfer.
When the energy reaches the special pair, that pair gets excited and ejects an electron. That electron is the start of the whole electron transport chain.
Two Photosystems, Two Jobs
Why two? Because a single photon doesn't carry enough energy to push an electron all the way from water to NADP⁺. The electron needs a boost — twice. So evolution split the job into two photosystems that work in series, like two pumps lifting water up a hill.
Photosystem II (PS II) comes first in the sequence. Its reaction centre chlorophyll absorbs best at 680 nm, so it's called P680. PS II is the water-splitting photosystem. It uses light energy to pull electrons from water, releasing oxygen as a byproduct. The excited electron from P680 is passed to a primary acceptor, and P680⁺ becomes the strongest biological oxidising agent known — strong enough to rip electrons from water.
Photosystem I (PS I) comes later. Its reaction centre absorbs best at 700 nm, so it's called P700. PS I receives electrons that have already lost some energy after passing through the electron transport chain from PS II. Light excites P700, and this second boost gives the electron enough energy to reduce NADP⁺ to NADPH.
Important
The numbering (I and II) is historical — PS I was discovered first, but PS II actually works before PS I in the electron flow. Think of it as: PS II → electron transport chain → PS I → NADPH.
The Precise Statement
A photosystem is a pigment-protein complex embedded in the thylakoid membrane, consisting of:
Antenna complex (light-harvesting complex): hundreds of pigment molecules that absorb photons and transfer energy to the reaction centre.
Reaction centre: a special pair of chlorophyll molecules (P680 in PS II, P700 in PS I) that undergoes charge separation upon excitation — losing an electron to a primary acceptor.
Photosystem II (P680) catalyses the light-driven oxidation of water:
2H2O4hνO2+4H++4e−
The excited electron from P680* reduces plastoquinone (PQ), which then carries the electron toward the cytochrome b₆f complex.
Photosystem I (P700) catalyses the light-driven reduction of NADP⁺:
NADP++H++2e−2hνNADPH
The excited electron from P700* reduces ferredoxin (Fd), which then reduces NADP⁺ via the enzyme ferredoxin-NADP⁺ reductase.
Splitting of water is associated with the inner side of the thylakoid membrane.
The water splitting complex is physically part of Photosystem II, which is itself located on the inner side of the thylakoid membrane.
Because of this location, the protons and oxygen produced when water is split are released into the lumen, the space enclosed on the inner side of the thylakoid, rather than toward the stroma. …
Water splitting is carried out by a complex associated with Photosystem II, which sits on the inner surface of the thylakoid membrane, so its protons and oxygen are released into the lumen.
The splitting of water answers a specific problem: Photosystem II continuously loses electrons to the electron transport chain, and these electrons must be replaced if PS II is to keep functioning. This replacement is achieved by splitting water — two molecules of water are split into four protons, oxygen, and four electrons, and the electrons released refill PS II. …
Method 1 — Locate where the water-splitting complex physically sits
Recall that splitting of water is a function tied to Photosystem II — not to Photosystem I, ruling out (a) and (c).
Recall that the water-splitting (oxygen-evolving) complex is physically embedded in the thylakoid membrane, on the side facing the lumen — i.e. on the inner surface of the thylakoid membrane. …
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
TG EAPCET 2022Set ap-2022-07-31-AN1 markMCQ
Q.Identify the respective absorption maxima of chlorophyll-a in reaction centres of photosystem-I and photosystem-II.
(A) 700 nm and 680 nm
(B) 680 nm and 700 nm
(C) 650 nm and 750 nm
(D) 750 nm and 650 nm
›Reveal solutionSolution
The reaction centers of Photosystem I and Photosystem II contain specialized chlorophyll-a molecules that absorb light maximally at 700 nm and 680 nm, respectively. The correct option is (A).
Concept and Intuition
Photosynthesis in plants involves two main photosystems, Photosystem I (PSI) and Photosystem II (PSII), which work in tandem to capture light energy. Each photosystem is a complex of proteins and pigment molecules, primarily chlorophylls and carotenoids. While many pigment molecules act as antenna pigments, collecting light and funneling it to a central point, a special pair of chlorophyll-a molecules forms the reaction center of each photosystem.
These reaction center chlorophyll-a molecules are crucial because they are the ones that actually convert light energy into chemical energy by undergoing photo-oxidation (losing an electron). Although both reaction centers contain chlorophyll-a, their specific protein environments within PSI and PSII cause slight differences in their electronic structure, leading to distinct absorption maxima. This allows them to absorb light most efficiently at slightly different wavelengths, optimizing the overall light capture process.
Step-by-Step Explanation
Photosystem I (PSI) Reaction Center:
The reaction center of Photosystem I is known as P700. The 'P' stands for pigment, and '700' indicates that this specific chlorophyll-a pair absorbs light most effectively at a wavelength of 700 nanometers (nm). This absorption maximum is due to the unique protein environment surrounding the chlorophyll-a molecules in PSI, which slightly shifts their absorption spectrum compared to free chlorophyll-a or chlorophyll-a in PSII.
Photosystem II (PSII) Reaction Center:
Similarly, the reaction center of Photosystem II is designated as P680. Here, 'P' again stands for pigment, and '680' signifies that this chlorophyll-a pair has its peak light absorption at 680 nanometers (nm). Like P700, this specific absorption characteristic arises from the particular arrangement and interaction of the chlorophyll-a molecules with the surrounding proteins within the PSII complex.